LECTURE XXIII. 



WHILE the procambial tracts are forming the protoxylem tubes, 

 the cells of the apex surrounding them are expanding rapidly. 

 Large vacuoles develop in their protoplasm, increasing their volume 

 at a quick rate. This expansion is chiefly in a longitudinal direc- 

 tion, and is probably to be attributed to the action of the osmotic 

 pressure of soluble substances in the protoplasm in pushing 

 out the plastic vertical walls of the cells. The water supplied by 

 the wood tubes leading up to the apex allows this osmotic pressure 

 to act. The elongation of these cells involves that of the protoxy- 

 lem tubes, and the longitudinal extension of their walls is 

 plainly indicated by the drawing out of their spiral thickenings and 

 the increased spacing of the rings. The most rapid elongation of 

 the apex takes place immediately after the differentiation of the 

 first protoxylem tubes. Consequently those which are nearest the 

 pith have the most widely spaced thickenings : those outside them, 

 being formed subsequently to the period of maximum growth, have 

 their spiral and annular thickenings closer together. 



The protoxylem tracheids being formed in the midst of soft un- 

 differentiated tissues are circular in cross section. Once secondary 

 developments have consolidated the woody cylinder the new 

 tracheids are developed under mutual pressure between the rigid 

 woody cylinder and the cohering cortex outside. The effect of 

 this pressure is seen in the fact that the secondary tracheids are 

 prismatic in form polygonal in cross section. 



The development of these tracheids takes place somewhat as 

 follows : each cambium cell is prismatic in form. Its long axis is 

 parallel to that of the stem. Its cross section is rectangular and oblong 

 and so oriented in the stem that its two longer sides are parallel 

 to the surface (tangential) while the two shorter sides seen in the 

 transverse section are radially disposed. These prismatic cells are 

 terminated above and below by a bevelled end. If you have 

 followed this description you will realise that the cambium cells 

 appear as small rectangles in transverse sections, as very long 

 rectangles in radial sections and as long parallelograms with sloping 



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